Control strips are particularly useful in monitoring the setting up of printing presses for new printing jobs by allowing careful comparison of the parameters of a printed product to the same parameters of a proof signed by a customer. Parameters which are compared include ink hues and densities, hues of secondary colors, dot gain/loss and halftone dot deformation as a result of slur or doubling. Control strips are also useful for maintaining printing parameters over a period of time to enable the printing of faithful copies of a proof of an original.
Conventional control strips for offset or gravure color printing include off-the-shelf control strips which cannot be modified by a user. A wide variety of such strips are sold, under tradenames such as Chromalin by Dupont or Matchprint by 3M, to cater to different printing needs. Often printing workshops are required to stock large numbers of control strips to cover ordinary printing jobs.
State of the art control strip patterns are described in the following documents:
Elyjiw, Zenon. "GATF Compact Color Test Strip", Research Project Report No. 6079, Graphics Arts Technical Foundation, Inc., Pittsburgh, Pa.: 1968.
GATF Quality Control Device Catalog, Graphic Arts Technical Foundation, Pittsburgh, Pa.: 1989.
The application of control strips in the conventional printing and proof process is now briefly explained. A control strip is manually mounted with a color separation film on a plate. The plate is exposed and then developed. This stage is performed for each color separation needed to print copies of the original. A color proof is produced in a similar fashion.
The plates are placed in a predetermined color sequence on a printing press, and the printing run is initiated. If the printing quality of output sheets is not satisfactory, as evidenced by comparison to the proof, then operation parameters of the printing press are changed accordingly to compensate for any deviations from the desired quality.
This technology presents various problems which have been discussed in the literature, such as the problem of mutual registration of color separations discussed in U.S. Pat. Nos. 4,469,025 to Loffler et al, automatic evaluation of the control strip discussed in 4,448,533, and in-process control to deal with the problem of variation between copies discussed in 4,852,485.
State of the art technology has eliminated the film stage by enabling direct exposure of plates from digital information stored in a color electronic pre-press system. This technology is known as computer-to-plate (CTP) technology. A commercially available device for performing CTP is the Raystar Imagesetter, commercially available from Scitex Corporation, Herzlia, Israel.
Similarly, a proofing process known as direct digital color proofing (DDCP) enabling direct exposure of proofs from digital information stored in 8 color electronic pre-press system has been developed and is known in the art. The Approval proofer, commercially available from Kodak, is an example of a DDCP device.
Postscript is a format defined by Adobe Systems Inc., California, U.S.A., for defining page parameters such as types of letters and graphics parameters of images for output by a printer or film imagesetter. Initially, Postscript was used for black and white page printing. Electronically mountable control strips for black and white Postscript defined pages are known and are discussed in "Digital Control Wedge", UGRA Mitteilungen, Vol. 1, April 1990. The output of this type of process is a printed page or film provided by an imagesetter or printer.